The present invention relates to a method for the remediation of media. More particularly, the present invention relates to dehalogenation of halogenated hydrocarbons in soil and water.
Chlorinated hydrocarbons are known to contaminate the soil and groundwater at thousands of sites throughout the United States and other parts of the world. Halogenated hydrocarbons are soluble in groundwater and can therefore be transported to drinking water reservoirs where they may pose serious health hazards. In many groundwater aquifers, halogenated hydrocarbons undergo only limited transformation and must therefore be removed prior to entry into drinking water.
Trichloroethylene, a volatile, chlorinated aliphatic hydrocarbon, is regarded as the most prevalent groundwater contaminant in the United States, being the most frequently reported contaminant at hazard waste sites on the National Priority List of the United Stated Environmental Protection Agency. The wide distribution of trichloroethylene can be attributed to its excellent solvent and degreasing properties which made it desirable for industrial applications. The use of trichloroethylenes became subject to regulation when they were found to be suspected carcinogen in mice. Trichloroethylene is one of the fourteen volatile organic compounds (VOC) regulated under the Safe Drinking Water Act Amendments of 1986. Other chlorinated hydrocarbons of concern include tetrachloroethylene, dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1-dichloroethane, chloroethane, carbon tetrachloride, chloroform, and dichloromethane.
Bioremediation is one of the methods used to stimulate in situ degradation of contaminants. For example, biodegradation of contaminants by indigenous microbial populations is common, and in many aerobic environments, the addition of nutrients to stimulate the growth of microorganisms can be an effective bioremediation tool in the cleanup of petroleum hydrocarbons. These processes rely on oxidative degradation under aerobic conditions, and the microbes use the contaminant itself as a carbon and energy source.
Anaerobic approaches to in situ bioremediation are generally thought to be less expensive and less invasive than aerobic approaches, largely due to the high cost and engineering challenge associated with the subsurface delivery of oxygen. In anaerobic environments, chlorinated solvents may be bioremediated in a process of sequential chloride removal called reductive dehalogenation. For example, tetrachloroethylene can be dehalogenated via this process to ethene, an innocuous end product, through the following sequence of intermediates. Tetrachloroethylene can be reduced to trichloroethylene. Trichloroethylene can be reduced to dichloroethylene. Dichloroethylene can be reduced to vinyl chloride. Vinyl chloride can be reduced to ethene.
In order for reductive dehalogenation to occur, an electron donor must be present to provide energy for growth and maintenance of the dehalogenating microorganisms. Conventional supplied materials used to support the bioremediation of chlorinated hydrocarbons have typically been soluble liquids, such as lactic acid, methanol, or molasses. These materials degrade or dissipate rapidly, and therefore require continuous or semi-continuous addition in order to sustain reductive dehalogenation. Use of these materials can also cause bacterial plugging of injection wells. Recently, lactic acid polymers have been developed which can be mixed with soils, place in monitoring wells, or injected into the subsurface. These viscous polymers break down slowly, releasing electron donor at a slower rate and sustaining dehalogenation over a longer period than soluble substrates. Subsequently, system design, construction, and operation and maintenance costs are much lower using these materials. However, the cost of commercial lactic acid polymer is very high. This is a significant problem because the major cost of anaerobic biodegradation systems utilizing slow release substrates lies in the material cost of the electron donor itself.
In order to bioremediate soil, microbial carriers have been studied. Kozaki et al. (European Patent No. 0 594 125 A2) discuss the use of natural polymers as a carrier for supporting microorganisms used in soil remediation, but not as an electron donor for the bioremediation process itself. In addition, Kozaki et al. are concerned with aerobic biodegradation, not with the anaerobic reductive dehalogenation of chlorinated hydrocarbons.
Generally, it is desirable to bioremediate contaminated soil without the need for continuous or semi-continuous addition. New techniques are constantly being sought to improve the bioremediation process which are both efficient and do not require constant reapplication.
The present invention provides a process for treating contaminated media which comprises adding at least one electron donor to contaminated media under anaerobic conditions to dehalogenate at least one halogen on at least one halogenated hydrocarbon in the presence of dehalogenating microorganisms wherein the electron donor comprises chitin, chitin-derivative, or combinations thereof.
A further embodiment of the present invention provides a method for enhancing the anaerobic biodegradation of at least one halogen on at least one halogenated hydrocarbon comprising the steps of:
optionally processing the chitin, chitin-derivative, or combinations thereof before exposing the chitin, chitin-derivative, or combinations thereof to the halogenated hydrocarbon, and
exposing the chitin, chitin-derivative, or combinations thereof to the halogenated hydrocarbon in the presence of dehalogenating microorganisms.
A further embodiment of the present invention provides a system for treating contaminated media comprising:
at least one electron donor comprising chitin, chitin-derivative, or combinations thereof wherein the electron donor is added to contaminated media under anaerobic conditions to dehalogenate at least one halogen on at least one halogenated hydrocarbon; and
dehalogenating microorganisms.